1. Field of the Invention
The present invention relates to an oscillation device that is utilized for a wireless terminal device of, for example, a mobile communication apparatus. Such an oscillation device is formed by including a voltage control oscillator for controlling the output phase by using, for example, a PLL circuit.
2. Prior Art
In recent years, major portions of high frequency wireless circuits have been implemented on semiconductors in wireless terminal devices of mobile communication apparatuses as a result of the pursuit of miniaturization. In such a circumstance, oscillators, which are the core parts of wireless devices, are also integrated on semiconductors. Differential type voltage control oscillators are mainly used as such oscillators.
A differential type voltage control oscillator has a circuit configuration as that shown in FIG. 4, which is characterized in that noise in common mode is small. In FIG. 4, symbol 401 indicates a power supply terminal. Symbol 402 indicates a constant current source. Symbols 403a and 403b indicate inductors. Symbol 403 indicates a differential tank circuit formed of inductors 403a and 403b. Symbol 404 indicates a control voltage terminal. Symbols 405a and 405b indicate varactor diodes. Symbol 405 indicates a differential variable capacitance circuit formed of varactor diodes 405a, 405b and control voltage terminal 404. Symbols 406a and 406b indicate oscillation transistors.
Though the operation of the above described differential type voltage control oscillator is described in detail in, for example, “RF Microelectronic” by Behzad Razavi, edited and translated by Tadahiro Kuroda, Maruzen, 2002, pp. 234-254, of which the detailed descriptions are omitted, differential variable capacitance circuit 405 formed of control voltage terminal 404 and varactor diodes 405a and 405b, as shown in FIG. 4, is used for the operation.
In addition, fixed capacitance switching circuit 508, as shown in FIG. 5, is further added to the configuration of the differential type voltage control oscillator shown in FIG. 4, and thereby, the bandwidth of the variable frequency is attempted to be widened. The same symbols as in FIG. 4 are attached to the corresponding parts in FIG. 5, of which the descriptions are omitted. Symbol 507 indicates a fixed capacitance switching control voltage terminal. In addition, symbols 508a and 508b indicate varactor diodes.
In the following, the operation of the differential type voltage control oscillator is described in reference to FIG. 6. In the graph of FIG. 6, the lateral axis indicates control voltage Vt, which is applied to control voltage terminal 404, while the longitudinal axis indicates the oscillatory frequency of the differential type voltage control oscillator.
FIG. 6 shows a change in the oscillatory frequency relative to control voltage Vt in the case where the fixed capacitance value is set at value A and value B, respectively, by switching the value of the fixed capacitance switching control voltage, which is applied to fixed capacitance switching control voltage terminal 507. As shown in FIG. 6, the band of the oscillatory frequency is switched in accordance with the switched fixed capacitance value A or B, and thereby, the oscillation band can be covered in a wide range.
In addition, a variety of attempts are made to maintain the signal purity of this differential type voltage control oscillator. A differential type PLL circuit as shown in, for example, Japanese Unexamined Patent Publication H7 (1995)-297710 “Improved Phase Lock Loop” is utilized, and thereby, it is attempted to be maintained a high signal purity, in spite of the existence of external noise and noise from the inside of the semiconductor. Such a differential type PLL circuit is described in “RF Microelectronics” by Behzad Razavi, edited and translated by Tadahiro Kuroda, Maruzen, 2002, pp. 271-301, of which the detailed descriptions are omitted.
A conventional differential variable capacitance circuit used for such a differential type PLL circuit has a configuration as shown in FIG. 7, in order to cope with the differential control voltage. In the following, the configuration and the operation of the conventional differential variable capacitance circuit are described in reference to FIG. 7. In FIG. 7, symbol 701 indicates a first differential terminal. Symbol 702 indicates a second differential terminal. Symbol 703 indicates a first capacitor. Symbol 704 indicates a second capacitor. Symbol 705 indicates a first varactor diode. Symbol 706 indicates a second varactor diode. Symbol 707 indicates a first resistor. Symbol 708 indicates a second resistor. Symbol 709 indicates a first control voltage terminal. Symbol 710 indicates a second control voltage terminal. Symbol 711 indicates a third resistor. Symbol 712 indicates a reference voltage terminal.
This differential variable capacitance circuit adds the differential control voltages on the plus side and on the minus side relative to the reference voltage that is supplied to reference voltage terminal 712 to control voltage terminals 709 and 710, respectively, so as to drive varactor diodes 705 and 706 by means of potential differences between the differential control voltages and the reference voltage, and thereby, can change the differential capacitance.
In the above described configuration, however, a problem arises where the differential balance of the capacitance is lost in the differential variable capacitance circuit, increasing noise in common mode in the oscillator and deteriorating the signal purity, in the case where a divergence occurs in the power supply voltage due to a change in the environmental temperature or the affection of switching of the power supply load.